This invention relates to thermal spraying, particularly to wire thermal spraying apparatus with two motors for feeding the wire, and more particularly to arc thermal spraying apparatus with two motors for feeding the wire.
Thermal spraying involves the feeding of a material through a heating zone to melt the material and propel it in finely divided form to a substrate to produce a coating. In one class of thermal spray guns, a wire tip is melted and atomized by a gas (usually air) at high pressure to produce the spray stream. The wire typically is a metal such as steel, nickel alloy, molybdenum, aluminum or zinc to effect coatings for repair, electrical conductivity, or resistance to wear, heat or corrosion.
Several means for heating may be utilized. One is with a combustion gun, for example as taught in the aforementioned U.S. Pat. No. 5,275,336 (Stasi et al.), wherein a single wire is fed axially through a nozzle into an annular ring of combustion jets such as an oxygen-acetylene flame which melt the tip. An air cap encompassing the nozzle and wire tip channels the annular flow of compressed air to atomize the melting tip and produce the spray stream.
Another type is an arc thermal spray gun in which two wires are fed into the gun and brought together at their tips in the spray head, for example as taught in U.S. Pat. No. 4,668,852 (Fox et al.) Electrical current passed via tubular electrodes contacting the wires near the tips effects an arc between the wires to melt the tips. An air cap encompasses the electrodes and wire tips so as to channel the annular flow of air to atomize the melting tips and produce the spray stream. An arc gun has a control unit where a power supply is contained. Arc current is fed to the gun electrodes via cables that may be incorporated in conduits guiding the wires to the gun.
Wire feeding is continuous and generally is effected by a light motor in the gun that, along with a wire engagement mechanism, pulls each wire from a wire supply such as a reel or barrel. The motor may be an air turbine or an electrical motor with an associated power supply and speed control. The electrical motor typically is DC with a silicon controlled rectifier (SCR) power supply which is capable of varying the motor speed. Often the single gun motor is sufficient. However, for longer distances from the wire supply, particularly in the case of arc guns, a second push motor is desired or even necessary to overcome loading in the conduits and pulling the wires from their supplies. A push-pull, two-wire arc thermal spray gun apparatus is disclosed in Japanese patent No. 1638109, and a push-pull one-wire welding apparatus is disclosed in U.S. Pat. No. 2,719,245. The push motor is mounted near the wire supply and has a wire engagement mechanism similar to that of the push motor. Settings of the push motor have been quite critical so as to provide enough pushing torque to take some of the loading from the pull motor, but not push so hard as to jam and bind the wire into the cable. Often there have been oscillations in the wire feeding without proper settings.
Regulated load sharing between two motors has been known for heavier duty applications than for thermal spraying. In one case three phase motors drive each end of a large printing roll to reduce torsion in the roll. The motors are identical, have a fixed spacing, and operate at constant speed. Paper width and thickness may be different at different times.
Another application has been in wire mills where rods are reduced in size by a series of motor-driven roll mills that operate at very high speed at the smaller diameter end. A larger size motor pulls rod through the mill, and a smaller motor feeds the wire into its end configuration. Distances are fixed. The material may vary from time to time, with correspondingly different motor settings. A further application has been for a belt drive, with identical motors and all other conditions remaining constant.
In these load sharing applications, one motor is maintained at a constant speed by conventional means such as armature voltage or tachometer feedback, or by constant speed AC operation. A signal representing loading torque of that motor is detected, being an armature current measurement in the case of a DC motor. The torque current signal is fed to the second motor to regulate its current and thereby adjust its torque in proportion to that of the first motor. Circuitry for load sharing for larger motors is shown schematically in "Application Notes--Issue 2" of Sprint Electric, section 2 page 2 (undated).